In many clinical situations, it is critical to determine whether a given tissue is viable following an ischemic episode. For example, in cardiology the decision to intervene by thrombolytics, percutaneous transluminal coronary angioplasty (PTCA), or coronary artery bypass grafting (CABG) is made largely on the assumption that the affected myocardium is viable and therefore will benefit from the procedure (Bonow, R. O. Identification of viable myocardium. Circulation 94:2674-2680, (1996), Hendel, R. C. and Bonow, Ro. O. Disparity in coronary perfusion and regional wall motion: effect on clinical assessment of viability. Coron. Art Disease 4(6):512-520, (1993)). Similarly, treatment of stroke patients is strongly influenced by available information regarding tissue viability Shimizu, T., Naritomi, H., Kuriyama, Y. and Sawada, T. Sequential changes if sodium magnetic resonance images after cerebral hemorrhage. Neuroradiology 34(4):301-304 (1992)). Thus, one of the most important issues regarding the management of patients with cardiovascular disease is knowledge of the location and extent of injured but viable tissue.
Extensive clinical experience has demonstrated that one of the best approaches for determining viability is to test for normal cell membrane function (Bonow, R. O. Identification of viable myocardium. Circulation 94:2674-2680, (1996), Hendel, R. C. and Bonow, R. O. Disparity in coronary perfusion and regional wall motion: effect on clinical assessment of viability. Coron. Art Disease 4(6):512-520, (1993)), i.e. to test for continued function of the Na.sup.+ -K.sup.+ pump. The most abundant natural isotopes of Na and K, .sup.23 Na and .sup.39 K, can be detected by magnetic resonance. In principle, it should be possible to use .sup.23 Na and .sup.39 K MRI to non-invasively examine cell membrane function and therefore viability. Specifically, we have recently shown in an animal model that .sup.23 Na image intensity is approximately 100% higher in non-viable compared to viable regions following reperfused myocardial infarction (Kim, R. J., Lima, J. A. C., Chen, E-L., Reeder, S. B., Klocke, F. J., Zerhouni, E. A. and Judd, R. M. Fast 23Na magnetic resonance imaging of acute reperfused myocardial infarction:potential to assess myocardial viability. Circulation in press:(1997)). Unfortunately, however, the in vivo .sup.23 Na and .sup.39 K MR signals are very small. The MR sensitivities for .sup.23 Na and .sup.39 K are only 9.2 and 0.051% of the .sup.1 H MR sensitivity and that the in vivo concentrations of these nuclei are approximately 1,000 times lower than the in vivo water proton concentration. The combination of these factors results in .sup.23 Na and .sup.39 K MR signals which are approximately 22,000 (1/4.63.times.10.sup.-5) and 2.1 million (1/4.73.times.10.sup.-7) times smaller than the standard .sup.1 H signal, respectively.
Despite the small MR signal, several groups have succeeded in producing in vivo .sup.23 Na images of humans Shimizu, T., Naritomi, H., Kuriyama, Y. and Sawada, T. Sequential changes if sodium magnetic resonance images after cerebral hemorrhage. Neuroradiology 34(4):301-304, (1992))-, Ra, J. B., Hilal, S. K., Oh, C. H. and Mun, I. K. In vivo magnetic resonance imaging of sodium in the human body. Magn. Reson. Med. 7:11-22, (1988), Granot, J. Sodium imaging by gradient reversal. J. Magn. Reson. 68:575-581, (1986); Granot, J. Sodium imaging of human body organs and extremities in vivo. Radiology 167:547-550, (1988), Katz, J. and Cannon, P. J. Use of sodium-23 for cardiac magnetic resonance imaging and spectroscopy. In: Cardiac imaging, edited by Marcus, M. L., Skorton, D. L., Schelbert, H. R. and Wold, G. L. Philadelphia: W. B. Suanders Co., 1991, p. 828-840; Perman, W. H., Turski, P. A., Houston, L. W., Glover, G. H. and Hayes, C. E. Methodology of in vivo humans sodium imaging at 1.5 T. Radiology 160:811-820, (1986), Hilal, S. K., Maudsley, A. A., Ra, J. B., Simon, H. E., Roschmann, P., Wittekoek, S., Cho, Z. H. and Mun, S. K. In vivo NMR imaging of sodium-23 in the human head. J. Comp. Assist. Tomog. 9(1):1-7, (1985), Winkler, S. S. Sodium-23 magnetic resonance brain imaging. Neuroradiology 32:416-420, (1990)). However, most of these groups have not attempted to apply recently-developed high speed gradient-echo imaging techniques to the .sup.23 Na or .sup.39 K nuclei. One reason for the lack of high-speed imaging studies of .sup.23 Na may be that many groups are interested in quantifying intracellular Na.sup.+ concentrations, for which long TR's to ensure full relaxation are desirable. In addition, high-speed imaging of nuclei like .sup.23 Na and .sup.39 K is very demanding on gradient hardware due to the low gyromagnetic ratios. Recent advances in gradient technology, however, may make this issue less significant.
Because the T.sub.1 and T.sub.2 relaxation times of .sup.23 Na and .sup.39 K are much shorter than those of .sup.1 H data taken from (Wolf, G. L. Contrast agents for cardiac MRI. In: Cardiac imaging, edited by Marcus, M. L., Skorton, D. L., Schelbert, H. R. and Wold, G. L. Philadelphia: W. B. Saunders Co., 1991, p. 794-810), (Kim, R. J., Lima, J. A. C., Chen, E-L., Reeder, S. B., Klocke, F. J., Zerhouni, E. A. and Judd, R. M. Fast 23Na magnetic resonance imaging of acute reperfused myocardial infarction:potential to assess myocardial viability. Circulation in press:(1997), and (Burstein, D., Litt, H. I. and Fossel, E. T. NMR characteristics of "visible" intracellular myocardial potassium in perfused rat hearts. Magn. Reson. Med. 9:66-78, (1989)) for .sup.1 H, .sup.23 Na, and .sup.39 K, respectively!, it is unlikely that direct application of fast imaging concepts derived from experience with proton imaging would result in optimal imaging parameters for .sup.23 Na and .sup.39 K imaging. There continues to be a need in the art, therefore, for improved methods for .sup.23 Na and .sup.39 K imaging.
The present invention provides numerically simulated various imaging strategies to maximize .sup.23 Na and .sup.39 K signal acquisition per unit time to understand the effect of the short relaxation parameters on the data collection. Then using the simulation results as a guide, in vivo 3D .sup.23 Na images of the human heart were acquired in 15 minutes on a modified 1.5 T clinical scanner. The results show that the application of high-speed gradient-echo imaging techniques combined with recent advances in gradient technology make .sup.23 Na imaging of the human heart practical.